1. Field of the Invention
The present invention relates to an air fuel ratio control apparatus for an internal combustion engine with a use of a NOx catalyst.
2. Description of the Prior Art
FIG. 8 is a schematic diagram showing a conventional air fuel ratio control apparatus for an internal combustion engine and the combustion chamber 15 as well as the air intake system and the ignition system are designed to allow the lean combustion. The air intake port 2 of an internal combustion engine 1 is connected, through a manifold 4 with a fuel injection valve 3 attached to each cylinder, with an air intake pipe which comprises an air cleaner 5, an air flow sensor 6 for detecting air intake quantity Qa, throttle valve 7 and ISC valve 8. The air flow sensor, which is of a type such as the Karman's vortex, is preferably used. The exhaust pipe 14, to which the air fuel ratio sensor 12 such as a linear fuel ratio sensor 12 such as a linear air fuel ratio sensor 12 as a means for detecting the excess air ratio .lambda. (air fuel ratio information) is disposed, is connected to the exhaust port of the internal combustion engine 1 through the exhaust manifold 11.
The exhaust gas purification catalyst device 13 comprises two catalyst of a ternary catalyst 13 and a occlusion catalyst 13b, and the ternary catalyst 13a is disposed on the more upstream side of the occlusion catalyst 13b. The ternary catalyst 13a has a function of oxidization of HC (hydrocarbon) and CO (carbon monoxide) as well as of reduction of NOx. Reduction of the NOx by means of the ternary catalyst 13 is promoted utmost with the theoretical air fuel ratio being in the vicinity of 14.7. The NOx occlusion catalyst 13b has a function of occluding the NOx at an excessively rich Oxygen atmosphere (lean air fuel ratio) and of reducing the NOx at the oxygen deficient atmosphere (rich air fuel ratio) with existence of HC and CO. As the NOx occlusion catalyst 13b, a catalyst consisting of the alkali earth metals such as Ca (calcium) and Ba (barium) and Pt (platinum) is used. A catalyst, which performs reduction of NOx exhausted to the atmosphere by utilizing a characteristics of adsorbing the NOx exhausted from the internal combustion engine 1 at an excessively rich oxygen state (oxidation atmosphere) and of reducing the adsorbed NOx at an excessive hydrocarbon (HC) state (reduction atmosphere), is already known.
In the internal combustion engine 1, an ignition plug is provided for igniting the mixture gas of air and fuel supplied to the combustion chamber 15 from the air intake port 2. 18 is a crank angle sensor for detecting the crank angle synchronization signal .theta. CR from the encoder being interlocked with the camshaft, 19 is a throttle sensor for detecting the throttle valve opening degree, 20 is a water temperature sensor for detecting the cooling water temperature TW, 21 is an atmospheric pressure sensor for sensing the atmospheric pressure Pa, and 22 is an intake air temperature sensor Ta. The rotation number Ne of the internal combustion engine is calculated from the time interval of generation of the crank angle synchronization signal .theta. CR detected by the crank angle sensor 18. Within the car room an ECU (Electronic Control Unit) which comprises unshown input output device, memory devices (such as ROM, RAM, non-volatile RAM) storing a number of control programs, central processing unit (CPU) and timer counter, is installed and it performs a synthetic control of the air fuel ratio control device including the internal combustion engine 1. Next, description on the operation of the aforementioned apparatus will be given. The NOx occlusion catalyst 13b is to occlude the NOx during the lean air fuel ratio control, but because of limitations imposed on the quantity to be occluded by the catalyst for a continuous lean combustion operation, when the quantity to be occluded comes out saturated, most of the NOx (nitrogen oxide) exhausted from the internal combustion engine 1 is exhausted to the atmosphere. Then, the timing of shifting to the rich operation from the lean operation gives rise to a problem because, before the occlusion quantity of the NOx occlusion catalyst 13b reaches a saturated quantity, the air fuel ratio control must be shifted to the one being operated at an ideal air fuel ratio or in the vicinity of that ratio and reduction of NOx at the rich air fuel ratio or at the theoretical air fuel ratio must be started.
Generally speaking, a method of performing the rich combustion operation after performing the lean combustion operation for a predetermined interval is employed. This method is exemplified such that the ECU 23 estimates quantity of NOx exhausted from the internal combustion engine 1, and the rich operation is performed when the quantity of the exhausted NOx reaches a predetermined valve. An example of estimation of the quantity of the exhausted NOx is disclosed by the Japanese Laid-Open Patent Application No. H7-305644 such that by obtaining an estimation value DN of exhausted NOx concentration as the air fuel ratio map and obtaining a compensation coefficient KIg as the ignition timing map and also likewise obtaining K1 depending on the EGR quantity and temperature as other compensation coefficient map, quantity of the exhausted NOx QNO was obtained from the intake air quantity Qa according to the following equation (1): EQU QNO=K1.multidot.KIg.multidot.Qa.multidot.DN (1)
According to the Japanese Laid-Open Patent Application NO. H7-305644, though the NOx exhaust quantity QNO is estimated from the equation (1) as above, because of a fluctuation of ignition timing the relation between the air fuel ratio A/F and the NOx exhaust concentration estimation value DN and also the relation between the EGR quantity and the exhaust concentration estimation value DN heavily change. As a consequence the estimation accuracy can not be improved without obtaining the NOx exhaust concentration estimation value DN depending on the ignition timing. Also a number of maps are required resulting in having the memory capacity increased.